Line interface apparatus and method for isolating data terminal equipment from the line

ABSTRACT

An improved isolation circuit for protecting data communication equipment from high voltages on a transmission line has two mixers. The first dual balanced mixer (DBMX) translates a data signal on the transmission line to a high frequency and is coupled with an isolation circuit to the second DBMX. The second DBMX then translates the output of the first DBMX to an isolated data signal with the same frequency content as the original data signal.

FIELD OF THE INVENTION

This application relates to data communication devices, including, butnot limited to, a line interface apparatus and method for isolating dataterminal equipment from the line.

BACKGROUND OF THE INVENTION

Data communication devices (DCDs) are used for data communicationsbetween terminals or computers over data lines such as dial-up anddedicated telephone lines. These DCDs include modems, terminal adapters,and digital service equipment. DCDs should be isolated from highvoltages that may occur on the data lines. Considering the damage thathigh voltage can cause the low voltage electronics residing in the DCDs,it is necessary to isolate the low voltage electronics from highvoltages. This requirement is set forth in equipment specifications suchas Underwriters Laboratory 1459 and Bellcore Technical ReferenceTR-NWT-001089.

The methods currently used for isolation include transformers andhigh-voltage capacitors. Both these methods are unsatisfactory. Lineisolation based on the use of transformers depends on the enamel coatingon the magnet wire windings to provide protection. This enamel coatingadversely affects the telecommunications signals due to the leakageinductance associated with wire to wire spacing. The transformers usedare relatively large and expensive components of the data communicationsequipment. Because high-voltage capacitors must provide a low impedanceto the data signal, they tend to be physically large.

An improved method of isolating DCDs from the data lines that will alsoreduce size and cost is desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a DCD connected to a transmission line and to a dataterminal.

FIG. 2 shows a line interface circuit.

FIG. 3 shows a schematic for a double-balanced mixer.

FIG. 4 shows details of a line interface circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to provide low cost and efficient isolation, a line interfacecircuit has a first mixer coupled to the line for receiving the datasignal and transforming the data signal to a high frequency data signal.A voltage isolation circuit coupled to the first mixer isolates the highfrequency data signal, thereby generating an isolated high frequencydata signal. A second mixer coupled to the voltage isolation circuit andthe data terminal equipment transforms the isolated high frequency datasignal to a low frequency isolated data signal. Electrical componentsusing this improved circuitry typically require approximately 5% less ofthe volumetric space as conventional methods. In addition, bothcomponent cost and manufacturing cost are reduced.

Referring to FIG. 1, a DCD 50 provides an interface between a dataterminal equipment (DTE) 40 and the transmission line 30. The DTE 40 maybe, but is not limited to personal computers, computer terminals,switched digital devices, and test equipment. The DTE 40 transfers datato/from the DCD via interface 60 which includes, but is not limited to,RS-232, V.35, and RS422 type interfaces. The DCD 50 includes a lineinterface circuit (LIC) 100 to interface data signals to the line 30through an impedance matching network 120 (if required) and dataprocessing circuitry to translate data between the LIC 100 and the DTEinterface 60. This configuration provides a communications link betweenthe DTE 40 and the transmission line 30.

As shown in FIG. 2, the LIC 100 connects to an impedance matchingnetwork 120 with connection link 122. The impedance matching network 120connects directly to the transmission line 30. The LIC 100 comprises afirst double-balanced mixer (DBMX) 124 which translates the data signalat connection link 122 to a higher frequency data signal. First DBMX 126is coupled to Isolation circuit 128 and provides high voltage isolationbetween first DBMX 126 and second DBMX 130 while presenting a lowimpedance path for the high frequency data signal, thereby generating anisolated high frequency data signal. Second DBMX 130 translates theisolated high frequency data signal to an isolated low frequency datasignal. The output of second DBMX 130 is coupled to processor interfacecircuit 132. The low frequency data signal has approximately the sameinformation and frequency spectrum as the data signal at connection link122. The process interface circuit (PIC) 132 provides interface betweenthe LIC 100 and the remainder of the DCD 134. The PIC 132 provides ahybrid type interface where the transmit and receive data signals of theDCD are combined to produce signals suitable for transmission on thetransmission line. The PIC 132 includes transmit signal conditioning anda receive equalizer for line loss compensation.

The schematic for the first DBMX 126 is shown in FIG. 3. First DBMX 126includes an intermediate frequency (IF) port 150, a local oscillator(LO) port 152, and a radio frequency (RF) port 154. The IF port 150 isconnected to the center taps of RF input transformers 156 and outputtransformer 158. Transformers 156, 158 are connected together via adiode bridge network 160. The LO port 152 connects to input transformer156 to provide a high frequency signal to modulate the signal present atthe IF port 150. RF port 154 connects to output transformer 158. Thefirst DBMX 126 translates the signal at IF port 150 to a high frequencydata signal at the RF port 154 by a frequency amount approximately equalto the input signal frequency present at the LO port 152. The first DBMX126 also translates an input RF signal to a lower frequency signal atthe IF port 150 by a frequency amount essentially equal to the inputsignal frequency present at the LO port 152. The second DBMX 130 is thesame configuration as the first DBMX 126 where the RF port connects toisolation circuit 128 and the IF port connects to processor interfacecircuit 132, as shown in FIG. 2.

The interface circuit is shown in FIG. 4. The transmission line 30 isconnected to the IF port 150 of the first DBMX 126 by means of animpedance matching network 200, which includes resistors 202, 204, 206.The impedance matching network 200 is used (if necessary) to match theline characteristic impedance to the input impedance of the first DBMX126. A typical application would require matching a 135 ohm transmissionline impedance to a 50 ohms IF port 150 impedance. In such anapplication, values for resistors 202, 204 would be 107 ohms and thevalue for resistor 206 would be 63 ohms. In some applications, the LOport 152 to IF port 150 signal leakage may require a filter capacitor208 across the IF port 150 to attenuate the LO frequency. A typicalvalue for filter capacitor 208 would be 15 nanofarads (nF) for an LOfrequency of 10 megahertz (MHz) and an IF input port impedance of 50ohms, which would present a low impedance at the LO frequency and a 3decibel frequency of approximately 200 kilohertz (KHz).

The first DBMX 126 of the LIC 100 is driven at the LO port 152 with a 10MHz oscillator 210. The oscillator 210 couples through a line isolationnetwork 212 which includes capacitors 214, 216. Line isolation network212 provides high voltage isolation between the oscillator 210 and theLO port 152 of the first DBMX 126. Typical values for capacitors 214,216 would be 15 nF with a voltage withstand rating of 2500 VDC (voltsdirect current) to provide high voltage isolation with a low impedance(1 ohm) signal path at the LO frequency of 10 MHz. The signal at IF port150 is translated up in frequency by the oscillator frequency at the LOport 152. The resulting high frequency signal is present at the RF port154. The high frequency signal at the RF port 154 is also translateddown in frequency by the frequency present at the IF port 150.

The isolation circuit 128 couples the high frequency signals between theRF port 154 and the second DMBX RF port 204. The isolation circuit 128includes capacitors 214, 216 and provides high voltage isolation betweenthe RF ports 154, 204 of the two DBMX's 126, 130. Typical values forcapacitors 214, 216 would be 15 nF with a voltage withstand rating of2500 VDC to provide high voltage isolation with a low impedance ofapproximately 1 ohm to the signal path for the carrier frequency of 10MHz.

The second DBMX 130 has a high frequency signal present at the secondDMBX RF port 204. This signal is translated down in frequency by anamount defined by the second DMBX LO port 218 frequency. The second DMBXLO port 218 is also driven by oscillator 210. Low frequency signals atthe second DMBX IF port 220 are also translated up in frequency by thesecond DMBX LO port 218 frequency and result as high frequency signalsat the second DMBX RF port 204. A filter capacitor 222 may be requiredas a shunt element at the input of PIC 132 to attenuate any leakagesignal at the oscillator frequency. A typical value for capacitor 222would be 15 nF, which presents a low impedance across the IF port 220 atan oscillator frequency of 10 MHz and provides an IF 3 dB bandwidth ofapproximately 200 KHz for an IF port impedance of 50 ohms.

The PIC 132 in FIG. 4 interfaces the second DBMX IF port 220 to thetransmit/receive circuitry 224 of the DCD 50. The PIC 134 includestransmitter conditioning, receiver equalizer, and a hybrid combiningnetwork for transmit and receive signal transmission on a single pairline.

By sourcing both DBMX's 124, 130 from a common oscillator 210, thesignal from the transmission line connected to the IF port 124 istranslated up in frequency, coupled to the second DBMX 106 via theisolation circuit 128, and translated down in frequency to the secondDBMX IF port 220 by the same frequency. This process retains the lowfrequency integrity of the input signal while providing a high voltageisolation barrier between the DBMX's 124, 130. Also, the signalpresented at the second DBMX IF port 220 is translated up in frequency,coupled to the first DBMX 124 through isolation circuit 128, and backdown in frequency by the same amount in the first DBMX 124. Thus, a lowfrequency signal is provided for the transmission line at the IF port150. Low frequency signal integrity is maintained with a high voltageisolation barrier.

We claim:
 1. A line interface circuit for coupling data terminalequipment to a line, the data terminal equipment receiving a data signalover the line, comprising:(a) a first mixer coupled to the line forreceiving the data signal and transforming the data signal to a highfrequency data signal (110); (b) a voltage isolation circuit coupled tothe first mixer for isolating the high frequency data signal, therebygenerating an isolated high frequency data signal, the voltage isolationcircuit comprising high-voltage capacitors connected between the firstmixer and the second mixer; and (c) a second mixer coupled to thevoltage isolation circuit and the data terminal equipment fortransforming the isolated high frequency data signal to a low frequencyisolated data signal.
 2. The line interface circuit of claim 1, thefirst mixer being a double-balanced mixer.
 3. The line interface circuitof claim 1, the second mixer being a double-balanced mixer.
 4. The lineinterface circuit of claim 1, the low frequency isolated data signalhaving a frequency spectrum that is almost identical to the frequencyspectrum of the data signal.
 5. The line interface circuit of claim 1,the first mixer further having a local oscillator input coupling, thelocal oscillator input coupling having an isolation means for isolationfrom an oscillator, the first mixer receiving an isolated high frequencysignal from the oscillator.
 6. The line interface circuit of claim 1,the second mixer further having a local oscillator input coupling, thelocal oscillator input coupling receiving a high frequency signal froman oscillator.
 7. A line interface circuit coupling data terminalequipment with a line, the line having a data signal, comprising:(a) amixer coupled to the line for receiving the data signal, the mixerhaving a means for transforming the data signal to a high frequency datasignal; (b) a plurality of high voltage capacitors connected between themixer and the conversion means; and (c) a conversion means forconverting the isolated high frequency data signal to an isolated lowfrequency data signal, the means for converting being coupled toisolation means.
 8. A line interface circuit for coupling data terminalequipment to a line, the data terminal equipment receiving a data signalover the line, comprising:(a) a first mixer coupled to the line forreceiving the data signal and transforming the data signal to a highfrequency data signal (110); (b) a voltage isolation circuit coupled tothe first mixer for isolating the high frequency data signal, therebygenerating an isolated high frequency data signal; and, (c) a secondmixer coupled to the voltage isolation circuit and the data terminalequipment for transforming the isolated high frequency data signal to alow frequency isolated data signal, the low frequency isolated datasignal having a frequency spectrum that is almost identical to thefrequency spectrum of the data signal.
 9. The line interface circuit ofclaim 8, where the voltage isolation circuit comprises high-voltagecapacitors connected between the first mixer and the second mixer. 10.The line interface circuit of claim 8, the first mixer being adouble-balanced mixer.
 11. The line interface circuit of claim 8, thesecond mixer being a double-balanced mixer.
 12. The line interfacecircuit of claim 8, the first mixer further having a local oscillatorinput coupling, the local oscillator input coupling having an isolationmeans for isolation from an oscillator, the first mixer receiving anisolated high frequency signal from the oscillator.
 13. The lineinterface circuit of claim 8, the second mixer further having a localoscillator input coupling, the local oscillator input coupling receivinga high frequency signal from an oscillator.
 14. A line interface circuitfor coupling data terminal equipment to a line, the data terminalequipment receiving a data signal over the line, comprising:(a) a firstmixer coupled to the line for receiving the data signal and transformingthe data signal to a high frequency data signal; (b) the first mixerhaving a local oscillator input coupling, the local oscillator inputcoupling having an isolation means for isolation from an oscillator, thefirst mixer receiving an isolated high frequency signal from theoscillator; (c) a voltage isolation circuit coupled to the first mixerfor isolating the high frequency data signal, thereby generating anisolated high frequency data signal; and (d) a second mixer coupled tothe voltage isolation circuit and the data terminal equipment fortransforming the isolated high frequency data signal to a low frequencyisolated data signal.
 15. The line interface circuit of claim 14, wherethe voltage isolation circuit comprises high-voltage capacitorsconnected between the first mixer and the second mixer.
 16. The lineinterface circuit of claim 14, the first mixer being a double-balancedmixer.
 17. The line interface circuit of claim 14, the second mixerbeing a double-balanced mixer.
 18. The line interface circuit of claim14, the low frequency isolated data signal having a frequency spectrumthat is almost identical to the frequency spectrum of the data signal.19. The line interface circuit of claim 14; the second mixer furtherhaving a local oscillator input coupling, the local oscillator inputcoupling receiving a high frequency signal from an oscillator.
 20. Aline interface circuit for coupling data terminal equipment to a line,the data terminal equipment receiving a data signal over the line,comprising:(a) a first mixer coupled to the line for receiving the datasignal and transforming the data signal to a high frequency data signal;(b) a voltage isolation circuit coupled to the first mixer for isolatingthe high frequency data signal, thereby generating an isolated highfrequency data signal; and (c) a second mixer coupled to the voltageisolation circuit and the data terminal equipment for transforming theisolated high frequency data signal to a low frequency isolated datasignal, the second mixer having a local oscillator input coupling, thelocal oscillator input coupling receiving a high frequency signal froman oscillator.
 21. The line interface circuit of claim 1, where thevoltage isolation circuit comprises high-voltage capacitors connectedbetween the first mixer and the second mixer.
 22. The line interfacecircuit of claim 1, the first mixer being a double-balanced mixer. 23.The line interface circuit of claim 1, the second mixer being adouble-balanced mixer.
 24. The line interface circuit of claim 1, thelow frequency isolated data signal having a frequency spectrum that isalmost identical to the frequency spectrum of the data signal.
 25. Theline interface circuit of claim 1, the first mixer further having alocal oscillator input coupling, the local oscillator input couplinghaving an isolation means for isolation from an oscillator (208), thefirst mixer receiving an isolated high frequency signal from theoscillator.